1. Field of the Invention
The present invention relates to a digital video cassette tape recorder (VCR) connected between a digital video signal transmitter and a digital video signal receiver, and more particularly to an apparatus for, and a method of, reproducing digital video signals at a varied speed in such a digital VCR.
2. Description of the Prior Art
Generally, NTSC broadcast signals are transmitted at a bit rate of about 200 Mbit/s. However, digital video signals as the next generation broadcast signals are transmitted at a bit rate of about 1.2 Gbit/s. For transmitting such digital video signals via the channel of VHF of about 6 MHz or via UHF, the digital video signals are typically compressed at a compression ratio of 5:1 or 10:1 in a digital video signal transmitter to have the form of a bit stream.
After being transmitted via the transmission channel, bit streams of data compressed by the digital video signal transmitter are recorded on a tape by a digital VCR. In a reproduction mode, the bit streams recorded on the tape are reproduced by the digital VCR and then decoded by a digital video signal receiver to be displayed on a screen.
Producing such a digital VCR on a commercial scale requires the capability of reproducing digital video signals recorded on the tape in normal and reverse directions at various speeds.
When a speed-varied travel mode or a reverse travel mode is selected in such a digital VCR having the above-mentioned functions, bit streams recorded on the tape are discontinuously reproduced. As a result, the-reproduced bit streams have the form of discontinuous data bursts which are different from that of the recorded bit streams.
This will be described in detail, in conjunction with FIGS. 1A to 1D.
When the speed-varied travel mode is selected, reproduction heads of the digital VCR travel along a trace across recording tracks on a tape. Assuming that the reproduction heads travel along a trace extending across a recording track Fi and a recording track Fi+1 adjacent to the recording track Fi, as indicated by an arrow in FIG. 1A, bit streams reproduced have an envelope including intervals A enabling a detection of digital video signals and intervals B disabling the detection of digital video signals, as shown in FIG. 1B. The intervals A and B are repeatedly generated in an alternating manner.
As shown in FIG. 1C, each of the detection enable intervals A includes an interval portion A1 enabling the detection of digital video signals and an interval portion A2 with digital video signal discontinuously reproduced.
Digital video signals reproduced after generation of a discontinuous point in each detection enable interval A are meaningless data, as shown in FIG. 1D. Such digital video signals can not be decoded by a digital video signal receiver.
In other words, a digital video signal reproduced from the tape is divided into slices, each of which is the basic synchronization unit. A variable length-coded digital video signal for every slice is divided into macroblocks again. Digital video signals for macroblocks involved in the discontinuous interval A2 can not be reproduced.
This will be described in detail, in conjunction with FIG. 1D. In a case wherein an i-th macroblock MBi and an i+1-th macroblock MBi+1 in a slice have been recorded on the tape in a continuous manner, the i+1-th macroblock MBi+1 involving the discontinuous interval A2 can not be totally reproduced, but is partially reproduced. When a digital video signal indicative of the i+1 macroblock MBi+1 is partially reproduced, (namely, the discontinuous interval A2 is decoded in the receiver,) error may occur. As a result, subsequent digital video signals form a meaningless bit stream which can not be displayed on a screen.
Accordingly, a variable length decoding for digital video signals is initiated when a decodable slice start code SSCi is detected and is until a digital video signal indicative of the i-th macroblock is detected. From this point, digital video signals reproduced from the tape are not decoded until the next slice start code SSCi+1 is detected.
However, the above-mentioned general digital VCR can not decode directly digital video signals of macroblocks reproduced from the tape. These reproduced digital video signals are decoded in the digital video signal receiver.
Although the digital VCR can detect every discontinuous interval, it can not detect the end of the i-th macroblock MBi just preceding the discontinuous interval.
For reducing error generated in the process of decoding reproduced digital video signals of macroblocks following the macroblock MBi+1 involving a discontinuous point, respective variable length-coded length information I1 to In of digital video signals of macroblocks MB1 to MBn included in each slice Si are recorded on corresponding slice headers SHi, as shown in FIG. 3, so that they are transmitted together with the digital video signals.
Using the coded length information I1 to In of digital video signals of macroblocks MB1 to MBn included in each slice Si, the digital VCR detects respective positions of macroblocks decodable by the digital video signal receiver. The digital video signals respectively corresponding to the detected positions are decoded by the digital video signal receiver 3.
In the above-mentioned system, however, there is a problem of a reduced compression rate of digital video signals because coded length information for all macroblocks are recorded on corresponding slice headers.